Make DAC structure packed

include/nuttx/analog/dac.h

@@ -47,6 +47,7 @@
  ************************************************************************************/
 
 #include <nuttx/config.h>
+#include <nuttx/compiler.h>
 
 #include <sys/types.h>
 #include <stdint.h>
@@ -77,7 +78,7 @@ struct dac_msg_s
 {
   uint8_t      am_channel;               /* The 8-bit DAC Channel */
   int32_t      am_data;                  /* DAC convert result (4 bytes) */
-};
+} packed_struct;
 
 struct dac_fifo_s
 {